Abstract

The Frieda River prospect is located between the Frieda and Lagaip fault zones, two major structural features of the New Guinea Mobile Belt in the West Sepik District, Papua New Guinea. The prospect can be subdivided into three spatially and geologically different areas: Frieda Complex, Mianmin area, and Nena Diorite area. The Frieda Complex is the remnant volcanic edifice of an island stratovolcano interstratified in the mid-Miocene Wogamush Formation. Intrusive and volcanic units in the complex are texturally similar andesitic hornblende-plagioclase porphyries and are probably comagmatic. Pyritic replacement copper-gold and porphyry copper deposits are associated with early and late phases, respectively, of extensive district-scale advanced argillic alteration along the central axis of the Frieda Complex. The Mianmin area is a separate, unmineralized volcanic center that contains igneous rocks similar to those of the Frieda Complex. The Nena Diorite, a composite, holocrystalline, intermediate to mafic body, intrudes Upper Cretaceous- to Eocene-age basement rocks. It is located north of, and separated from the Frieda Complex by, the Frieda Fault.Igneous activity in the Frieda River prospect extended over an interval of at least 4 m.y. On the basis of field relations and K-Ar ages, this igneous activity is subdivided into early intrusion (17.3 to 16.8 m.y. ago), main intrusion (16.3 to 13.1 m.y. ago), and late alteration (about 12.8 to 11.2 m.y. ago) events. K-Ar ages on alunite appear to be too young to be those of formation of this mineral, and the oldest (13.0 + or - 0.4 m.y.) is interpreted as a minimum age for the Nena pyritic replacement copper-gold deposit. Geothermal activity, which caused district-scale alteration, extended from earlier than 14.0 + or - 0.4 m.y. until about 11.2 + or - 0.2 m.y. ago. The discordance of about 2 m.y. between K-Ar ages of coexisting hornblende and biotite in two diorites is thought to register the cooling history of the hydrothermal system. A K-Ar age of 13.6 + or - 0.4 m.y. was obtained on hornblende from a sample of Horse microdiorite, regarded as the progenitor of porphyry copper mineralization. This age is interpreted as near that of emplacement and provides an older limit for the age of vein-controlled transitional alteration associated with most porphyry copper mineralization. The primary biotite apparent K-Ar age of 11.5 + or - 0.2 m.y. from the same sample is accepted as a younger limit for the age of porphyry copper mineralization.K-Ar ages from the Mianmin area and the Nena Diorite area are very similar to those found for dioritic intrusions in the Frieda Complex.Intrusive and volcanic rocks of the Frieda River prospect exhibit limited compositional variation; they are all of andesitic composition and belong to a normal K calc-alkaline suite. The three geologically distinct areas of the prospect are distinguishable on the basis of rock and mineral chemical differences. Textural and chemical features of the Freida Complex and Mianmin area igneous rocks are compatible, with much of their diversity in lithology and dispersion in chemistry being due to their representing cumulate-melt mixtures. They are thought to have crystallized from hot (>900 degrees C), oxidized magmas which were emplaced at a shallow level in the crust (P total < 0.5 kb).

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